We went during Presidents Day this year. From Sunday 2/18-Tuesday 2/20. We didn't arrive until about 11am on Sunday and it was pretty crowded at Islands of Adventure when we arrived. We only did Suess Landings and then I let the kids play in the Jurassic Park area. We left after we ate at Mythos. But only because I was really tired and wanted to lay down.
On Monday and Tuesday, we arrived at opening and the parks were EMPTY unitl about 11-12. We got pretty much everything done and only went on things that had reasonable wait times later in the day.
Although it got crowded later, the parks did thin out again around 4pm and the last couple hours were good.
But it wasn't bad. Just get there early to do the most popular rides and do shows when it gets crowded.